When a hydrocarbon fuel gas such as town gas is used as a reaction gas to be introduced into a fuel cell, a fuel reformer is required to reform such a hydrocarbon gas (raw fuel gas) into a hydrogen rich gas.
In the fuel reformer, steam is mixed with the raw fuel gas to yield a mixed gas, and thereafter hydrogen is generated, for example, by allowing these two gases to react with each other in a high-temperature environment set at 350° C. or higher.
Specifically, in the reforming reaction (steam reforming), first, methane as the raw fuel gas and steam are allowed to react with each other to yield hydrogen and carbon monoxide, and further, the carbon monoxide thus generated and steam react with each other to yield hydrogen along with carbon dioxide.
These reactions are represented in terms of reaction formulas as follows:CH4+H2O→H2+COCO+H2O→3H2+CO2 
The above-described reforming reaction is an endothermic reaction, and for the purpose of performing a reforming reaction with such a satisfactory conversion rate that the residual amount of methane is 1% or less, it is necessary to heat a reforming catalyst in a fuel reformer at least to 640° C., and preferably to 700° C. or higher. Accordingly, an internal reforming fuel cell is designed in such a way that the thermal energy required for the reforming reaction is obtained from the high temperature exhaust gas discharged from a fuel cell stack (see, for example, Patent Document 1).
Incidentally, for example, in a solid oxide fuel cell, when it is a high-temperature operation type operated at temperatures in the vicinity of 1000° C., it is relatively easy to recover the thermal energy required for fuel reforming; however, when it is a low-temperature operation type operated at temperatures in the vicinity of 700° C., it becomes difficult to sufficiently recover the heat required for the endothermic reaction because the discharged thermal energy is smaller than that in the aforementioned high-temperature operation type. When sufficient energy is not supplied to the fuel reformer, the reforming reaction becomes insufficient, leading to a fear that no hydrogen rich reformed gas is obtained.
When a large amount of methane is contained in the reformed gas due to insufficient reforming, the carbon in the reformed gas is deposited in the power generating cells to drastically degrade the cell performance, and it interferes with efficient power generation.
Patent Document 1: Japanese Patent Laid-Open No. 2005-19034